Is it the turbo spinning down?
FWIW, I hear strange noises all the time, and it takes me a while to realise that they are normal... every once in a while, some different noise will stand out... sometimes riding with a tail wind I'll hear all sorts of stuff that I'm not used to hearing and it will totally freak me out. Usually it's nothing.
Personally, for a new whining type of noise, I'd think about the valve train.